Safety footwear is used, and often required, in many industries, for example the construction industry. Such safety footwear may incorporate a protective insole or a protective toe cap, or both. A typical example would be a safety boot, which example will be used throughout this specification although the structures and principles described are equally applicable to shoes and other types of footwear.
In a typical safety boot a steel sole plate overlays a large portion of the outsole of the boot to prevent penetration of the sole by sharp objects such as nails and the like. A conventional sole plate, comprising a unitary forepart plate, shank and heel, is formed from steel. In a conventional construction, the sole plate can be cemented to the upper which has been formed over a last (lasted); it can be riveted to the insole at the rear and floated into the outsole material; or it can be cemented to the insole board prior to lasting.
These conventional constructions provide a number of disadvantages. Attachment of the protective plate, insole board and sockliner, being three separate components, requires three separate operations. Typically the plate is attached to the insole board by one of the methods described above, the insole board is attached to the upper, and the sockliner is inserted after construction of the boot is otherwise complete.
Moreover, where the sole plate is cemented to the lasted upper or floated into the outsole material, injection molding of the outsole does not result in complete filling, leading to a void in the area under the plate resulting in a soft sole.
It is desirable in such a construction that the insole board be affixed directly to the outsole at the periphery, to prevent separation, and thus the sole plate is cut smaller than both the insole board and the outsole, leaving a margin around which the latter can be tacked or cemented together. The smaller sole plate provides a margin for attachment of the outsole to the upper. For this reason a conventional sole plate covers only approximately 70% to 80% of the sole of the boot, leaving a margin vulnerable to penetration.
The sole plate should be rigid in the shank and heel regions of the sole, since these do not flex during normal use. On the other hand, considerable flexing occurs during normal use along a line transverse to the foot at approximately the ball of the foot. Conventional steel sole plates encounter problems with cracking along the region of flexure due to work hardening of the steel, which decreases the protective ability of the sole plate and can deform the contour of the sole. Cracks can open in the plate and protection is lost in these areas.
The present invention overcomes these disadvantages by providing an integral protective sole comprising a protective layer sandwiched between an insole board and a fabric liner. In both preferred embodiments described herein the protective layer is formed by injection of molten plastic between the sockliner and the insole board, in the process bonding the sockliner and insole board to opposite sides of the protective plastic layer as an integral unit and thus avoiding the need for the additional steps of cementing and tacking to affix the separate components as described above.
The use of plastic injection molding to form the protective layer further permits both the protective layer and the insole board to form to the desired shape under heat and pressure, in a single step, and the shape of the insole board is thereafter maintained by the hardened plastic.
Since the insole board forms the lower layer of the insole, the outsole can be bonded directly to the insole board, obviating the need to leave a margin around the protective plate and allowing for complete filling of the outsole when molded.
The plastic layer according to this design provides full coverage over the sole, thus avoiding an unprotected margin which is vulnerable to penetration by sharp objects. Furthermore, the use of plastic as a protective layer, while equally effective to steel in puncture resistance, results in greater flexibility and durability particularly in critical regions such as along the ball of the foot.
The use of a plastic protective layer, dispensing with the need for a thick and rigid steel plate in the heel and shank regions of the sole, results in a much lighter protective insole unit than a conventional insole composed of steel sole plate, insole board, sockliner, tacks and assorted cements.
The present invention thus provides a protective insole for safety footwear comprising a protective layer composed of plastic and including a flexible forepart portion, an insole board bonded by the plastic to a bottom surface of the plastic, and a fabric liner bonded by the plastic to a top surface of the plastic.
The present invention further provides a method of constructing a protective insole for safety footwear comprising the steps of cutting a fabric liner and an insole board to the desired shape, placing the fabric liner and the insole board into a mold allowing for a clearance between the liner and the insole board, and injecting molten plastic through an injection port in the insole board to fill the clearance between the liner and the insole board, whereby upon hardening of the plastic the liner and insole board are bonded to the plastic to form an integral protective insole.